The present invention relates to motor-vehicle tires and specifically to tires having a radial carcass and suitable for medium and/or high class motor-vehicles in which tires affording high performance are required. More particularly, the invention relates to the tread pattern in said tires.
It is known that tires have a thick tread band disposed on the crown of the carcass in which a relief pattern exhibiting longitudinal and/or transverse grooves is formed by molding, said pattern having the function of giving the tire some driving behavior features such as in particular good capability of traction and roadholding at any ground conditions, that is when the road is dry, wet or covered with snow, and irrespective of the ride conditions, that is to say on rectilinear paths or in case of drift, i.e. when there are bends or side thrusts for example due to the effect of wind.
As regards roadholding on wet ground, longitudinal grooves are of the greatest importance in that, due to said orientation, they succeed in discharging the water collected in the tire contact-area, thereby ensuring the wheel/road contact and avoiding the well-known and dangerous aquaplane phenomenon.
For the above reason all tires except some special tires intended for machines having a very low ride speed and therefore unaffected by aquaplane phenomenon, are always provided with wide continuous longitudinal grooves which are oriented circumferentially.
On the contrary, as regards the tires capability of traction, this is connected with the presence of transverse grooves in the tread and for this reason, in the absence of said grooves, the longitudinal ones generally have a zigzag course so as to exhibit inclined edges relative to the vehicle ride direction.
The efficiency of the transverse grooves as regards the capability of traction becomes increasingly greater as the inclination of said grooves approaches the axial direction: on the contrary, this axial inclination is negative as regards the possibility of discharging the water picked up from ground because this orientation makes it difficult to pump the water in the longitudinal grooves under the contact-area, and also as regards the directional stability of the tire above all at the high speeds achieved with present motor-vehicles.
Therefore the attempts to reach the best compromise between the different requirements of modern tires have brought to the development of both symmetric and asymmetric tread patterns relative to the diametrical plane of the tire, having rectilinear circumferential grooves and transverse grooves all inclined in the same direction or alternately in the opposite direction with respect to the axial direction of the tire: the so-called "helix-shaped" and "fish bone-shaped" patterns are well-known and widely spread and represent an example of the foregoing.
Upon increasing of the performance required from tires as regards power and speed, the longitudinal grooves have shown a new drawback, specifically in tires having a radial carcass.
Radial carcass tires, usually referred to as "radial tires", are those provided with a carcass the reinforcing cords of which substantially lie in radial planes, that is containing the rotational axis of the tire, and an annular reinforcing structure which is circumferentially inextensible (the so-called belt) disposed on the crown of the carcass, inserted between the carcass and the tread band and ensuring dimensional stability of the tire, particularly in the circumferential direction, as well as the necessary drive behaviour qualities.
In fact a tire provided with a radial carcass alone, due to its structure, would not be dimensionally steady under the effect of the efforts due to inflating pressure and those connected with the centrifugal force in use, and would substantially be incapable of offering good driving features, in particular directional and transverse stability.
It is known that roadholding in case of drift and the transverse stability of the radial tire closely depend on the strength of the belt structure to axially-acting tangential efforts transmitted between the tire and the road; on the other hand, it has been found that in tires suitable for vehicles offering high power and speed performance said strength is also conditioned by the geometry of the tread pattern.
In particular, the structural strength of the belt is generally limited due to the fact that, at the longitudinal tread grooves, where the tread thickness is clearly reduced to a minimum value, the belt has a preferential deformation area, that is a "hinge". This hinge, when the tire in use is subject to side forces, triggers a tire of stress causing the dynamic yielding of the belt structure, which gives rise to a decrease in roadholding and general behavior qualities of the tire and consequently of the vehicle at high speed.
In order to overcome these drawbacks tread patterns devoid of longitudinal grooves have been so far proposed, only having transverse grooves inclined to both axial and circumferential directions of the tire, optionally emerging in the axial direction on the tire shoulders, such as for example the tread described in British Patent GB 2,229,974.
However tires provided with this type of pattern, as well as those mentioned above and exhibiting a helix-shaped pattern suffer the drawback of developing, under rectilinear ride conditions, a drift force of high strength and greatly changing on varying the torque applied to the tire, which hinders the driving and, without varying the steering angle, causes the vehicle to slide under acceleration and braking conditions, bringing about an uncontrolled change in the trajectory thereof. For the purpose of restraining this undesirable tire behavior, attempts are usually made to balance this drift force by mounting the tires on the vehicle in a particular manner, that is with the tread patterns disposed symmetrically to the vehicle axis.